22.10: Inner vs. Outer Planets

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What's better than a field trip?

All of the inner planets are orbited by man-made satellites. Jupiter and Saturn have man-made satellites too. We can see what the planets look like from the photos they take. The satellites also carry instruments that collect a lot of important data. This is better than taking a field trip to such hostile places. You can learn a lot while sitting on your own, very comfortable planet!

The Inner Planets

The four planets closest to the Sun—Mercury, Venus, Earth, and Mars—are the inner planets or terrestrial planets (Figure below). They are similar to Earth. All are solid, dense, and rocky. None of the inner planets has rings. Compared to the outer planets, the inner planets are small. They have shorter orbits around the Sun and they spin more slowly. Venus spins backward and spins the slowest of all the planets.

All of the inner planets were geologically active at one time. They are all made of cooled igneous rock with inner iron cores. Earth has one big, round moon, while Mars has two very small, irregular moons. Mercury and Venus do not have moons.

The relative sizes of Mercury, Venus, Earth, and Mars

This composite shows the relative sizes of the four inner planets. From left to right, they are Mercury, Venus, Earth, and Mars.

The Outer Planets

Jupiter, Saturn, Uranus, and Neptune are the outer planets of our solar system. These are the four planets farthest from the Sun. The outer planets are much larger than the inner planets. Since they are made mostly of gases, they are also called gas giants. Pictured below are the relative sizes of the outer planets and the Sun (Figure below).

The relative sizes of the Sun, Jupiter, Saturn, Uranus, and Neptune

This image shows the four outer planets and the Sun, with sizes to scale. From left to right, the outer planets are Jupiter, Saturn, Uranus, and Neptune.

The gas giants are made mostly of hydrogen and helium. These are the same elements that make up most of the Sun. Astronomers think that most of the nebula that formed the solar system was hydrogen and helium. The inner planets lost these very light gases. Their gravity was too low to keep them, and they floated away into space. The Sun and the outer planets had enough gravity to keep the hydrogen and helium.

All of the outer planets have numerous moons. They also have planetary rings made of dust and other small particles. Only the rings of Saturn can be easily seen from Earth.

Summary

The four inner planets have slower orbits, slower spin, no rings, and they are made of rock and metal.
The four outer planets have faster orbits and spins, a composition of gases and liquids, numerous moons, and rings.
The outer planets are made of hydrogen and helium, so they are called gas giants.

Review

Describe the characteristics of the four inner planets.
Describe the characteristics of the four outer planets.
Why are the inner and outer planets so different from each other?